docs: def round_edge_grill(alpha, l=None, Dh=None, fd=None):
      r'''Returns the loss coefficient for a rounded square grill or square bar
      screen or perforated plate with rounded edges of thickness l, as shown in
      [1]_.
  
      for Dh < l < 50D
  
      .. math::
          K = lookup(alpha)
  
      else:
  
      .. math::
          K = lookup(alpha) + \frac{fl}{\alpha^2D}
  
      Parameters
      ----------
      alpha : float
          Fraction of grill open to flow [-]
      l : float, optional
          Thickness of the grill or plate [m]
      Dh : float, optional
          Hydraulic diameter of gap in grill, [m]
      fd : float, optional
          Darcy friction factor [-]
  
      Returns
      -------
      K : float
          Loss coefficient [-]
  
      Notes
      -----
      If l, Dh, or fd is not provided, the first expression is used instead.
      The alteration of the expression to include friction factor is there
      if the grill is long enough to have considerable friction along the
      surface of the grill.
      alpha must be between 0.3 and 0.7.
  
      Examples
      --------
      >>> round_edge_grill(.4)
      1.0
      >>> round_edge_grill(.4, l=.15, Dh=.002, fd=.0185)
      2.3874999999999997
  
      References
      ----------
      .. [1] Blevins, Robert D. Applied Fluid Dynamics Handbook. New York, N.Y.:
         Van Nostrand Reinhold Co., 1984.
      '''
      t1 = float(splev(alpha, grills_rounded_tck))
      if Dh and l and fd and l > 50*Dh:
          return t1 + fd*l/Dh
      else:
          return t1